The present invention relates generally to operational amplifier, and more particularly, to an improved low noise input stage of an amplifier circuit.
It is generally desirable to include a current cancellation circuit in operational amplifiers for the purpose of cancelling the DC bias current associated with the input signal. One such cancellation circuit is shown in U.S. Pat. No. 3,714,600 to Kuijk et al. This patent shows the use of a constant current source and a measuring transistor arranged at the base and collector of an input transistor respectively. A current proportional to the collector-emitter current of the input transistor is supplied from the constant current source to the base of the input transistor, resulting in reducing the DC bias current flowing into the input stage. Although the DC bias current cancellation is achieved in Kuijk, the noise current spectral densities associated with the DC bias and cancelling currents statistically add due to the random nature of noise, thereby increasing the over all noise spectral density associated with the input signal. The increase in noise density at the input stage degrades the input signal and therefore decreases the accuracy of the amplifier output. Additionally, the inclusion of a measuring transistor in the path of the AC signal of the amplifier further results in a degradation of the AC performance of the amplifier.
Another circuit commonly used for bias current cancellation includes a base current generator connected to a current mirror having a one-to-one ratio. A current proportional to the DC bias current of the input signal is provided by the current generator to the input of the mirror, whereby an equivalent current is reflected at the mirror output. This output current is then combined with the amplifier input signal for reducing the input bias current associated with the same. Although the two DC currents cancel out algebraically in this circuit design, the noise current spectral densities of the two DC current signals add together. As in Kuijk et al., the adding of the noise currents spectral densities results in producing an even potentially larger noise level of the input signal.
The noise associated with these DC currents derive from a plurality of sources in analog devices. Shot Noise Current results in the space charge layers by carriers being repelled through the electric field. Frequency Dependent Noise Current and Burst Noise are noise representations attributed to the sporadic nature of thermal generation and recombination. It is the combination of these noise currents that result in the degradation of the input signal to the operational amplifier.
It is, therefore, an object of the present invention to provide an amplifier circuit with a reduced noise current spectral density associated with the cancellation current.
It is further an object of this invention to provide a DC input bias current cancellation circuit which provides substantial reduction in noise current spectral density (NCSD) of the input signal while cancelling out the DC bias currents of the input signal.
It is also an object of this invention to provide a method for producing an input DC bias cancellation current having a low noise current spectral density.
These and other objects of the invention are attained in an amplifier having an input bias current cancellation circuit comprising a first means for providing a DC current of an amplitude K times greater than the amplitude of the DC bias current, a second means for reducing the output current of the current source by K times, and a third means for cancelling the DC bias current of the input signal with the reduced current.
In the preferred embodiment, the first means includes a current source and the second means includes a current mirror having a ratio of 1/K. By providing a current greater than the DC input bias current to the input side of the current mirror, the output current of the mirror is equivalent to the DC bias current for cancelling the same, but with the quality of having reduced noise spectral density associated with the cancelling current. This provides a substantially lesser amount of noise to add with the noise inherent with the DC input bias current, and thus, reduces the overall noise spectral density of the input stage of the amplifier.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.